The way I think of it is that the numbers in the matrix are the points gained for X therefore the points lost for Y, Y would want to minimise the points lost so playing Y1 and risking losing 10 points isn't worth it.

I guess you just change the signs for each entry to make it a matrix for player Y and do exactly what you would for player X.

Sorry if that doesn't make much sense, I'm only just getting my head round this myself